Problem:
 +(0(),x) -> x
 +(1(),-(1())) -> 0()
 -(0()) -> 0()
 -(-(x)) -> x
 -(+(x,y)) -> +(-(x),-(y))
 +(+(x,y),z) -> +(x,+(y,z))
 +(x,y) -> +(y,x)

Proof:
 Open